1. Field of the Invention
The present invention relates to a liquid crystal composition and a liquid crystal display, and particularly it relates to a smectic liquid crystal composition which expresses a ferroelectric phase (hereunder abbreviated as "SmC* phase") or an antiferroelectric phase (hereunder abbreviated as "SmC.sub.A * phase") etc., and to a liquid crystal display which employs the smectic liquid crystal composition.
2. Description of the Related Art
Recently, liquid crystal cells have become widely used to take advantage of their characteristics such as thinness, light weight, low electricity consumption, etc., and most of these liquid crystal cells employ nematic liquid crystals.
Since nematic liquid crystals are driven based on the anisotropy of the dielectric constant of the liquid crystal, their speed of response is slow and they are therefore in need of improvement.
In contrast, liquid crystal cells employing liquid crystals exhibiting the chiralsmectic C phase (hereinafter abbreviated as "SmC* phase"), which is ferroelectric and was discovered by Meyer et al., have a high speed response and memory characteristics which are not attained by nematic liquid crystals. Thus, application of ferroelectric liquid crystals to ferroelectric liquid crystal cells, because of these characteristics, is being actively researched.
However, the good orientation and memory characteristics required for liquid crystal cells are difficult to realize in a real cell using the above-mentioned ferroelectric liquid crystals, because ferroelectric liquid crystals are not resistant to external shocks, etc. and thus have many remaining problems to be solved.
On the other hand, Chandani, et al. have recently discovered an antiferroelectric phase (hereinafter abbreviated as "SmC.sub.A * phase") which exhibits three stable states at the low temperature end of the SmC* phase. These antiferroelectric liquid crystals exhibit a thermodynamically stable phase wherein dipoles are arranged in anti-parallel at every adjacent layer, and this causes a field-induced phase transition between the antiferroelectric phase and the ferroelectric phase which is characterized by a clear threshold value and double hysteresis characteristics in response to an applied voltage. New methods of indication are beginning to be investigated using this switching behavior.
Liquid crystal compounds with antiferroelectric phases are already known as described in Japanese Unexamined Patent Publication Nos. 1-213390, 1-316339, 1-316367 and 2-28128, and with disclosures of new antiferroelectric liquid crystal compounds, their numbers are ever increasing.
In practical terms, most of the antiferroelectric liquid crystal compounds produced to date have high melting points and temperature ranges for the antiferroelectric phase which are much higher than room temperature. Normally, the liquid crystal materials used for ferroelectric or antiferroelectric liquid crystal cells are mixtures of 5 to 10 different types of liquid crystal compounds. The reason for this is that It is currently impossible to achieve satisfactory characteristics required for liquid crystal cells by using only one type of liquid crystal compound, while the required characteristics can be satisfied by using a mixture of liquid crystal compounds with different physical characteristics, to achieve a balance of characteristics as a whole.
Incidentally, one of the problems involved in applying smectic liquid crystal compositions including ferroelectric liquid crystal compositions and antiferroelectric liquid crystal compositions to liquid crystal cells is disordered orientation due to temperature changes in the liquid crystal cell.
When the temperature changes from a high to a low temperature, the phase of the liquid crystal composition generally changes from smectic A phase (SmA phase).fwdarw.smectic C* phase (SmC* phase).fwdarw.smectic C.sub.A * phase (SmC.sub.A * phase).fwdarw.smectic I* phase (SmI* phase) or smectic I.sub.A * (SmI.sub.A * phase). In accordance, the layer spacing of the layer structure in the smectic phase of the liquid crystal composition is drastically reduced from the SmA phase, the reduction becoming more moderate by the SmC.sub.A * phase and reaching a minimum at a certain temperature, but then when the temperature is further lowered, it increases again.
On the other hand, when the temperature is raised from a low temperature to a high temperature, the layer spacing undergoes a change opposite to the one described above. The disordered orientation occurs when a cold/hot temperature cycle is applied to the liquid crystal cell.
The reason is believed to be that structural defects occur in the smectic phase layer to absorb the changes in layer spacing during the cold/hot temperature cycle, and thus create the disordered orientation. As a result of this disordered orientation which results in shifting between the liquid crystal optical axis and the polarizing plate angle, light leakage occurs during dark indication periods, thus lowering the value for the contrast represented by the brightness ratio of light indication to dark indication and becoming an obstacle to achieving a clear screen display. In addition, the disordered orientation also tends to produce burning. Burning of the display is a phenomenon caused by the difference in orientation, or brightness, resulting from application of different voltages, such as white indication voltage and black indication voltage, to picture elements with disordered orientation.
For example, as an example of a ferroelectric liquid crystal with temperature dependency of the layer spacing, the display disclosed in Japanese Unexamined Patent Publication No. 2-40625 employs ferroelectric liquid crystals which undergo no change of the layer spacing between the crystalline phase and the smectic phase after crystallization.
However, according to studies by the present inventors, this ferroelectric liquid crystal composition also undergoes variation of the layer spacing with temperature changes within the temperature range of the smectic phase, and thus it has been found that temperature changes within that range cause the disadvantage of disordered orientation and thus a poor indicating function as a liquid crystal cell.
The present inventors have therefore posited that if the layer spacing of smectic liquid crystals is roughly constant throughout temperature changes, then layer structure defects in the smectic phase, as well as disordered orientation, will be suppressed.
Furthermore, as mentioned above, in the case of antiferroelectric liquid crystals, the required physical characteristics cannot be satisfied when only one type of antiferroelectric liquid crystal compound is used to form the liquid crystals in the liquid crystal cell. For example, many liquid crystals have practical problems, such as a temperature range of the antiferroelectric phase which is much higher than room temperature, or a long response time for changes between negative polarity and positive polarity when the driving voltage of the liquid crystal is 50 V or greater. Also, with only one type of liquid crystal compound even the initial orientation is such that the liquid crystal molecules fail to be oriented in the rubbing direction, or even when oriented they have a high degree of light leakage during dark indication periods, thus lowering the contrast.
On the other hand, in the case of liquid crystal compositions which are mixtures of many liquid crystal compounds, the state of orientation is improved, thus lowering the light leakage during dark indication periods.
Consequently, liquid crystal cells are preferably not constructed using only one type of antiferroelectric liquid crystal compound, and as already mentioned they usually require use of an antiferroelectric liquid crystal composition which is a mixture of at least a few types of antiferroelectric liquid crystal compounds. The same also applies to ferroelectric liquid crystals. Because they are used for liquid crystal cells, the compositions must have a low tendency to disordered orientation.
In light of these circumstances, it is an object of the present invention to provide a liquid crystal cell or liquid crystal display which exhibits good contrast by minimizing disordered orientation occurring with temperature changes within a wide range and which has no burnout and high reliability, as well as a smectic liquid crystal composition for use in the liquid crystal cell or liquid crystal display.